Even More Sinkholes

19-Feb-2024

Then there were three four ...

Two more sinkholes have opened up within five days of each other along the route of tunnelling for High Speed 2’s (HS2's) Chiltern tunnels... HS2’s working assumption is that a combination of disturbance from tunnelling, and potentially recent rainfall, weakened pre-existing weak spots (New Civil Engineer)

However, the first Frith Hill sinkhole appeared a week or so after the lead TBM (Florence) passed under the field, and the 2nd (Cecilia) has just entered the area; has the recent rain found its way into the disturbed ground surrounding the tunnel ?

(photo - Mike Johnstone

And another ... are they related ?

The ~~new~~ first sinkhole, just off Frith Hill, with the North Portal works beyond

The latest sinkhole opened up a few days ago, within sight of the end of the tunnel - not far from Sibley's Rise, or Frith Hill.

Going Down Again - Hyde Heath

23-Dec-23: Now filled, path reopened

11-Nov-2023

Photo - Gareth Williams

Another 'small area of ground movement' - this time beside (under ?) a footpath on the north side of Hedgemore Wood, off Hyde Lane. Councillors visited the site on Wednesday 8th (according to the BFP) , and posted an account on 'X' on 10th Nov about a visit 'earlier today' - saying they were notified 'yesterday' - which would be the 9th.

Bucks councillors reported that "On 8 November, before the sink hole appeared, the data from the TBM indicated that the machine could be passing through a geological feature." so it seems likely that the hole appeared on the 9th, and was first reported ( via X) on the 10th. It is not clear why it falls to Bucks councillors to report the incident, or what grounds they have to be so reassuring. The hole is above the northern tunnel, bored by Florence, the lead TBM; Cecilia is around 350m behind and yet to pass through the area. We would advise against picnics in Hedgemore Wood next week.

Any statement from HS2 is (as yet) unknown to Google - perhaps they are checking the hole for horses or pedestrians, before saying anything ?

13-Nov-2023

Statement from HS2 ! (Tales of the expected ??)

Our Tunnel Boring Machine (TBM) Florence is tunnelling under the edge of a woodland, straddling a bridleway. There is a known geological fault in this area. Through our assessments we identified the need for additional monitoring which we put in place in line with our established procedures. Yesterday morning (9th Nov) whilst monitoring the area, a ground movement was observed. For safety reasons, the public bridleway has been closed off, the area is fenced off, and we are continuing tunnelling operations. We are engaging with key stakeholders, including the landowner/ leaseholder and Buckinghamshire Council. We will continue to keep you updated as we go forward.

(Private communication - nothing yet on the HS2 website )

11-Aug-2023

Sinking in Shardeloes

Ruislip Mudbath - Here

(Photo - Stuart King)

1-Aug-2023 - Plans for reinstatement

In accordance with these access agreements, Align will carry out a photographic pre-construction survey of the whole route before any remediation works begin and our environmental experts will check trees in the historic parkland to ensure that the access route from the temporary compound to the ground movement area avoids Root Protection Areas (RPA) and adequate separation from any trees with nesting bats or owls etc. All grazing livestock will be removed from the field by the farmer during the period when works are taking place and Align will provide traffic marshals for the duration of the works to manage vehicle movements and pedestrians on the Public Right of Way. (More...)

Preparation works:

Installation of a temporary compound, stockpile area and welfare unit
Define a route for of tracked vehicles only between the temporary compound and the area of ground movement to deliver backfill materials that does not impact trees in the parkland

Two-stage remediation works:

Planned to start week commencing 21 August: Removal of existing topsoil and subsoil material from the base of the ground movement feature followed by a structural backfill of the area (approximately 150-200 cubic meters) to approximately 400mm below the existing ground level, using chalk material from one of the local vent shaft sites at Little Missenden or Amersham.
Dates to meet planting season be confirmed: Secondary landscape remediation will follow after the ground has stabilised. The 400mm layer of subsoil and topsoil will be reinstated and tied into the existing grassland and the whole area reseeded. The fence around the site will be retained until the grass establishes. The timings for this are dependent on the seeding season.

After the temporary access, welfare security, plant and materials deliveries have all been arranged, the first stage remediation works will take approximately two weeks and then the site cabins, welfare facilities and protective matting will all be removed. The second stage will follow as a separate operation at a later date and will take approximately one week.

These works are to restore and reseed the area and we do not expect there to be any long-term effect on the landscape or visual impacts of the scheme. Appropriate archaeological monitoring will be undertaken throughout the works following a Written Scheme of Investigation (WSI) that will be prepared in consultation with Historic England, Buckinghamshire Council’s archaeologist and the Gardens Trust.

We will follow the HS2 Code of Construction Practice (CoCP) and carry out these works between 08:00 to 18:00 Monday to Friday and 08:00 to 13:00 on Saturdays and use appropriate pollution prevention measures like silt fencing to manage any surface water runoff.

18-July-23

HS2 have produced a document to answer frequently asked questions about ground movement and the sinkhole near Little Missenden,

8-Jun-23 Question to the Transport Minister, from Sarah Green

I’m pleased that earlier today the Transport Minister agreed to meet with me to hear concerns about the oversight of HS2 ...
I will happily (??) sit down with her & her constituents, to take her through what's been found (Mark Harper)

24-May-23

How it happened - statement from the Chiltern Society, following a meeting with HS2 and the contractors.

19-May-23

A spokesperson for HS2 Ltd said: "We are aware of a small (?) area of ground movement within a field above the Chiltern tunnels. Investigations are ongoing, but this is likely to be linked to pre-existing ground conditions above the tunnels." (Bucks Herald)

Would those be the pre-existing ground conditions we warned about in 2010 ? In a week or two, this could be underneath the River Misbourne - but its only a chalk stream, so who cares ? -
"as it is above a portion of completed tunnel, the HS2 spokesperson has confirmed it has had no impact on current tunnelling operations, which are continuing."

"current thinking is that it is probably a pre-existing dissolution feature or fissure which has partially collapsed". However, an industry specialist has suggested that the ground collapse could be linked to the tunnelling work.(No, really ? New Civil Engineer )

The Chiltern Conservation Board is alarmed ...

EA to the rescue ??

An Environment Agency spokesperson said: “We are investigating a sinkhole ... to understand whether it has resulted in any pollution to the local environment." (Bucks Free Press)- but not to see if the tunnel represents a threat to the Misbourne ?

Mr Strachan (QC for HS2, at the Select Committee Hearings) quoted from a letter from the Environment Agency –
‘Before we can approve applications in line with the protected provisions within the Hybrid Bill, we’ll need to be satisfied that all potential risks to the river and surrounding environment have been mitigated, and this will need to be supported by evidence from your ground investigation programme and we’ll continue to provide advice to ensure the proposed mitigation will be acceptable and approvals can be issued.’
So, Mr Smart, in general terms you’re referring to the protected provisions, but the EA is identifying here that they will continue to provide an approvals process before anything occurs, to ensure the protection of the River Misbourne and the water supply in the area, is that how it would work ? (Evidently not ... see MEP)

Other reports - BBC, ITV, Guardian, Chiltern Society. Video - Stuart King